The present disclosure pertains to control system design and operation when the controlled system includes multiple control targets with multiple constraints.
In a control system having more than one primary target to be controlled for multiple primary control objectives, such as thrust, fan operability, core operability, etc. (for a jet engine, for example), the control system will have multiple inputs and multiple outputs to control. Such a control system should address the challenge of multi-variable control with multiple constraints, particularly when the primary control objectives have high transient and dynamic requirements. The challenge fundamentally is a coordinated control to maintain primary control objectives as much as possible while enforcing a selected set of active constraints that can satisfy all potentially active constraints.
Traditionally, single-input-single-output (SISO) control is used for one primary control objective—for example in a gas turbine engine, fan speed only. The concerned constraints are converted to the control actuator rate—fuel rate, respectively, the constraint demanding most fuel rate is selected as most limiting constraint and enforced. Here, there is an assumption that fuel rate is always proportional to fan speed change, and fan speed changes always align up and dominate the thrust response and operability. This may be true in many operating conditions, but it is not true for certain operating conditions, such as supersonic operating area for conventional engine applications, not to mention non-conventional engine applications, such as powered lift operation.
Multiple constraints may be in one subset only, that is, at same time, only one primary controlled output needs to be traded off. There are cases, however, in which multiple constraints are in two or more subsets that require two or more primary controlled outputs to be traded off. Certainly, at most, the number of the subsets should be equal to the number of the primary control handles. For example, in the gas turbine engine example, if both “maximum core speed” and “maximum exhaust temperature” constraints are active, it may be necessary to trade off both primary controlled outputs, “fan speed” and “pressure ratio,” for better thrust and operability performance, while enforcing both the “maximum core speed” and “maximum exhaust temperature” constraints. It is a challenge to control multiple variables with higher dimension multiple constraints.
Previous approaches to solve this problem have either greatly oversimplified the problem or added substantial complexity. The oversimplified approach ignored fundamental confounding in the relationships between the controlled plant inputs and the performance trade-off and control mode selection decisions that must be made. This limited its applicability to certain 2×2 multi-input-multi-output (MIMO) systems, and does not represent a robust solution for higher dimension MIMO systems. The overly complicated approaches coupled the constraint control with the primary control, usually lost expected control objectives priority, and sacrificed the physical meaning, robustness, deterministicness, and maintainability of the control solution.